1. Field of the Invention
The present invention relates to a device for generating percussion sound pulses inside a well or a cavity and more particularly an emitting device adapted to be lowered to a certain depth in a well drilled in the ground, for generating sound pulses which may be used for carrying out seismic prospection operations for example.
A well pulse source has a certain number of advantages. By using such a source seismic prospection operations may be carried out from well to well which allows a high power of resolution to be obtained or else oblique seismic profiles to be rapidly plotted by picking up the emitted waves using a plulrality of receivers disposed on the surface.
It is furthermore more efficient than a source disposed on the surface. An appreciable part of the sound energy produced thereby is in fact emitted in the form of unusable surface waves or Rayleigh waves and the part of this energy which is emitted in the form of P or S type waves is considerably attenuated by the surface layer or altered layer (W.Z). The P or S wave energy yield of a well source emitting under the altered layer is therefore much higher.
2. Description of the Prior Art
Among well sources of a known type may be mentioned those which use solid explosives or explosive gas mixtures. Such sources produce very powerful seismic pulses but the frequency spectrum of these pulses comprises an appreciable part of high frequencies which have no practical interest in the field of seismic prospection. The power of the explosions generally results in damaging the well or impairing the neighboring regions, to such an extent that subsequent repetition of the seismic shots fired at the same places is difficult. Explosive sources are described for example in U.S. Pat. Nos. 3,408,622, 3,858,176.
Sources of the airgun type may also be mentioned in which seismic pulses are generated by sudden gas discharges inside the well filled with water. Such sources produce powerful sound waves but for operation thereof require the use of a compressor or compressed fluid reservoirs which, because of the reduced space of the drilled hole, must be installed at the surface and connected to the source by piping. Such a source is described for example in French Pat. No. 2 311 322.
A known device for generating sound pulses by percussion inside a drill hole comprises a cylindrical body provided with anchorage spikes actuated by hydraulic systems. The means for producing shocks are formed by two weights associated with coils which move with respect to each other and collide under the action of the electromagnetic forces generated in the coils by controlled electric currents. Such a device, which is described in French Pat. No. 2 442 455, is suitable for producing relatively weak transverse seismic pulses and thus is not suited for seismic prospection operations where the depth of the ground to be explored is generally very great.